1. Field of the Invention
The present invention relates to an antenna apparatus and a communication apparatus using the antenna apparatus, and more particularly relates to an antenna apparatus used in a mobile communication apparatus, and a communication apparatus using the antenna apparatus.
2. Description of the Related Art
Recently, amid advances in high-performance mobile telephones such as PHS, there are demands to provide a mobile communication apparatus wherein an antenna mounted thereon has even higher performance characteristics, such as high gain and a capability to be miniturized.
FIG. 6 shows a conventional antenna apparatus mounted on a mobile communication apparatus. In FIG. 6, an antenna apparatus 1 comprises a ground electrode 3 provided on one major surface 2a of a mount substrate 2, the corner of the major surface 2a having a removed portion 3a, a supply wire 4 provided in the removed portion 3a, and a surface-mount antenna 5 mounted thereabove. Then, the supply wire 4 is connected to a supply terminal (not shown in the diagram) of the surface-mount antenna 5, and also to a signal source 6 provided on the mount substrate 2.
FIG. 7 shows the surface-mount antenna 5 in more detail. The surface-mount antenna 5 has the basic constitution of that disclosed in Japan Unexamined Patent Publication No. 10-13139.
In FIG. 7, the surface-mount antenna 5 comprises several electrodes provided on the surface of a rectangular substrate 10, which comprises an insulating dielectric such as ceramic or resin. Firstly, a strip-shaped radiation electrode 11 is provided extending from the other major surface 10b of the substrate 10, across one end face 10d, and returning once again to the other major surface 10b. One end of the radiation electrode 11 is an open end 11a, and the other end connects to a first ground terminal 12, which extends from an end face 10c of the substrate 10 to a first major surface 10a thereof. Furthermore, a feeding electrode 13 is isolated from the radiation electrode 11 and is provided on a second major surface 10b of the substrate 10. One end of the feeding electrode 13 connects to a feeder terminal 14, which extends from the end face 10c of the substrate 10 to the first major surface 10a thereof. Similarly, a ground electrode 15 is provided on the other major surface 10b of the substrate 10 near the open end 11a of the radiation electrode 11. One end of the ground electrode 15 is connected to a second ground terminal 16, which extends from the end face 10c of the substrate 10 to the first major surface 10a. Furthermore, terminals for securing 17 and 18 are provided on the first major surface 10a of the substrate 10, and connect to the radiation electrode 11.
When the surface-mount antenna 5 is mounted on a mount substrate (not shown in the diagram), the five electrodes comprising the first and second ground terminals 12 and 16, the feeder terminal 14, and the terminal for securing 17 and 18, are connected by soldering to ground electrodes, supply lines, and electrodes for securing on the mount substrate side. Therefore, the five electrodes are represented as terminals in order to distinguish them from these other electrodes.
In a surface-mount antenna 5 having such a constitution, when a high-frequency signal is input to the feeder terminal 14, the high-frequency signal is transmitted to the radiation electrode 11 via a capacitance created between the open end 11a of the radiation electrode 11 and the feeding electrode 13. The radiation electrode 11 is made to resonate by the inductance component of the radiation electrode 11 itself, and by the capacitance formed between the open end 11a of the radiation electrode 11 and the feeding electrode 13. The surface-mount antenna 5 functions as an antenna by radiating a portion of the resonance energy into space as electric waves.
At this point, since one end of the radiation electrode 11 is an open end 11a, and the other end is a grounded end, the resonance is approximately one-quarter wavelength. A current 19 flowing to the radiation electrode 11 (i.e. the current flowing to the surface-mount antenna 5) flows from the grounded end of the radiation electrode 11 (more specifically, the first ground terminal 12) toward the open end 11a. As a result, magnetic field components of the electric waves radiated from the surface-mount antenna 5, and the electric waves received by the surface-mount antenna 5, vibrate mainly in the direction of the 2 axis of the coordinates shown in FIG. 7.
The electrodes and terminals of the surface-mount antenna 5 are provided separately from each other, but it is not absolutely necessary for them to be separated by some kind of clear boundary. The electrodes and the terminals may acceptably be provided together.
In the antenna apparatus 1 shown in FIG. 6, when the current 19 flows to the surface-mount antenna 5, an image current 7 which is 180 degrees out of phase with the current 19 flows mainly to the ground electrode 3.
However, when the antenna apparatus 1 is used, the mount substrate 2 is covered by a case of plastic, metal, or the like. The case is often left in such places as on a shelf, or in a bag. When placed on a shelf or in a bag, the case often comes into close contact with conductive objects comprising metal and the like, and such objects function more or less like ground conductors. This leads to a problem that, depending on the external environment in which this type of antenna apparatus 1 is used, the ground conductors in close proximity thereto alter the direction and position of the flow of the image current 7 of the current 19 flowing to the surface-mount antenna 5, affecting the characteristics of the antenna apparatus 1 such as its gain and directivity.